This invention relates generally to steam turbines, and more particularly to methods and apparatus for reducing stress in turbine buckets.
During operation, turbine buckets are exposed to centrifugal loads, which may induce vibratory stresses in the bucket, cause fatigue in the bucket and/or premature failure. Centrifugal loading may be a function of bucket operating speed, a weight of the bucket, and/or a location of the bucket relative to an engine centerline. As such, one known method of increasing the turbine bucket lifespan includes reducing the weight of the bucket. Additionally, the use of a hybrid bucket design allows for longer last stage buckets that equates to steam turbine output as the area increases. Also, the hybrid construction allows for more aerodynamic (wider chord) airfoils that improve stage efficiency. Lastly, the hybrid construction creates damping in the bucket/stage thereby improving the frequency response of the stage thereby improving reliability.
For turbine buckets or blades, vibratory stresses generally increase when these loads and stresses approach bucket natural resonant frequencies. The magnitude of the vibratory stresses when a bucket vibrates in resonance is proportional to the amount of damping present in the system (wherein damping includes material, aerodynamic and mechanical components) and the stimulus level. For continuously coupled buckets, the frequency of vibration is a function of the entire system of blades, and not necessarily that of individual blades.
In at least some known turbine bucket designs, the weight of the bucket is reduced by fabricating the bucket with hollow pockets which are then filled with a composite or polymer material. The filler material may comprise a polyimide or another type of polymeric resin (or combinations thereof) with continuous glass, carbon, KEVLAR® or other fiber reinforcement to achieve a composite matrix with the original airfoil surface. The pockets reduce the weight of the bucket while the fill material facilitates maintaining the profile and/or strength of the bucket. Composite matrix are now being designed to be used in units that have high bucket temperatures during windage conditions (low flow, high speed “wind milling” of buckets). However, such designs often lack sufficient adhesive bonds between the metal of the turbine bucket and the composite material. Specifically, composites capable of withstanding the engine's high temperatures generally adhere poorly to the bucket metal because the composite material weighs more than the polymer filler material.
For example, U.S. Pat. No. 5,720,597, entitled “Multi-Component Blade for Gas Turbine,” describes gas turbine aircraft blades constructed of metal and foam are provided with a composite skin, an erosion coating, or both. Configurations are disclosed that are applicable to fan blades, and more specifically to “propulsion engines.” As such, the sizes and shapes of the pockets are significantly limited. Moreover, U.S. Pat. No. 6,139,728, entitled “Poly-Component Blade for a Steam Turbine,” discloses configurations similar to those disclosed in U.S. Pat. No. 5,720,597, but for steam turbines. Benefits described include lower weight, which allows less robust blade alignment and thereby reduces cost. However, frequency tuning and damping benefits are not mentioned. Furthermore, U.S. Pat. No. 6,042,338, entitled “Detuned Fan Blade Apparatus and Method,” describes a “propulsion engine fan” and various types of blades with different pocket locations, but does not disclose blades of essentially one pocket with different rib structures. In addition, the disclosure is limited to pockets with radial location from a tip to 5%-38% span and chord wise from 15% to 35% from the leading edge and 20% to 45% from the trailing edge with similar limitations on the second or alternative pocket design.